For example, if you take that battery: https://www.solaris-shop.com/trojan-sind-06-1225-solar-industrial-flooded-6v-1225ah-battery/
Capacity: 1225Ah/6V 7.35 kWh
Weight: 190 kg
Price: 1222 USD
Cycle life: 3600@50% DoD
Then
1222 USD/(7.35 kWh x 0.5 x 3600)=0.092 cents per kWh
Looks suprisingly competitive actually if you dont't mind the weight.
But does anybody have any information on the Sunica.plus flooded NiCd batteries Saft advertises for solar use? Too exotic and too expensive I guess?
That seems to be at half MSRP in therms of cost but fair enough as it is available at that price.
The 3600 cycles at 50% DOD means using it that way every day for 10 years and then you ignored the calendar aging that can be very significant in solar energy storage application's where battery most of the time is less than fully charged.
Also that capacity is for 100h discharge rate but if you want to assume a 50% DOD every day then best case charge and discharge need to happen in less than 10h each.
If you then look at 10h capacity it is just 835Ah significantly less than at 100h
Then if you also consider calendar aging and also the fact that you need a significantly oversized battery capacity for your application compared to LiFePO4 and also the much lower charge discharge efficiency.
They mention a service life of 17 years based on IEC61427 and if you check what that means here
https://www.trojanbattery.com/pdf/RE_IEC_61427_Standard.pdfthey consider that 50 + 100 shallow cycles are equivalent with 1 year of service life.
The first 50 cycles are 30% DOD and the 100 cycles at 25% DOD with very low discharge rates just 0.1C (10h discharge rate).
For simplicity I will be generous and say 150 cycles at 30% DOD x 17 years as the max possible life for the battery again excluding most of the calendar aging as that is not considered in this tests that only take a few months.
$1222 / 150 x 0.3 x 17 x 5kWh = $0.32/kWh way to optimistic as calendar aging is not considered in the IEC61427
Now if you do the same for LiFePO4 and ignore calendar aging you get
300 to $400/kWh cost and 3000 to 6000 cycles at 100% DOD but I will take a Winston cell that is just 5000 cycles at 80% DOD so
$400 / 5000 cycles x 0.8 = $0.1/kWh again if you include calendar aging the realistic number I always use is $0.2/kWh.
My battery is made with A123 cells (I fully offgrid) and I use the battery heavily at an average 47% equivalent DOD every day and after first year measured degradation was 0.9% from that around 0.3% is from cycling and 0.6% is from calendar aging. This cells are rated 5400 cycles at 100% DOD.